Many display apparatuses display images on a display panel by use a light unit which comprises a backlight for illuminating variable light transfer pixels of a pixilated display panel. Usually, the pixilated display is a matrix display. Typically, the backlight provides a non-varying light spectrum and the input image is reproduced by modulating the optical state of the pixels such that the light transmission is modified to provide the desired intensity (intensities) for the pixel. Backlight sources have conventionally predominantly been provided by the use of fluorescent lamps. However, Light Emitting Diodes (LED's) have also been proposed for backlights. LEDs can provide almost monochromatic spectra and LED backlights are often used to provide a multi-colored backlight. A known transmissive Liquid Crystal Display (LCD) comprises pixels made of liquid crystal material of which an optical transmission is controlled in accordance with the image to be displayed. In another known reflective Digital Mirror Device (DMD) display, the pixels comprise small mirrors, which can tilt and where an angle of the tilt of the mirrors is controlled in accordance with the image to be displayed. Transflective displays, which partly reflect and partly transmit light from the light sources, are also known.
In a color display device, each one of the pixels comprises sub-pixels and associated color filters to obtain different colors that together provide the color of the pixel in accordance with the image to be displayed. The colored lights that leaves the color filters and which illuminate the associated sub-pixels are often referred to as the primary colors of the color display device. These primary colors define the color gamut that the display device can display.
Traditionally, color display devices have used three primary colors, such as typically Red (R), Green (G) and Blue (B). As a consequence, input images are typically defined in a three-component color space, which usually is the RGB color space or a color space related thereto. Recently, so called multi-primary displays have been introduced which use more than three primary colors. It should be noted that the term “colors” is used as a convenient term for light sources with different spectra that are not necessarily (but may be) substantially monochromatic. Such displays are also referred to as wide gamut displays because a wider color gamut can be displayed by using at least four instead of three primary colors.
Power consumption is one of the most important parameters of both low-end and high-end displays. Indeed, power consumption is an important issue in display apparatuses and much research has been undertaken to develop techniques for reducing the power consumption. Power consumption can be reduced not only in the backlight unit (light source efficiency and design, as well as driver electronics), but also by introducing different pixel layouts in the panel. One approach that has been proposed for a wide gamut display is to use four sub-pixels per pixel wherein one of the sub-pixels is white. Usually, the other sub-pixels are red, green and blue, but other colors are possible, such as a saturated or desaturated yellow, cyan, a second blue etc.
For the same backlight intensity, the extra white sub-pixel (which has a substantially transparent color filter) has a much higher luminance than the other sub-pixels because the color filters between the light source and the other sub-pixels suppress a large part of the spectrum. Consequently, the power consumption can be minimized by providing the white part of the color via the white sub-pixel instead of via the other sub-pixels of the pixel. The transparent color filter need not be actually provided but often is present unintentionally because the light leaving the light source has to travel a predetermined distance through the transparent material covering the white sub-pixel.
Thus, an efficient RGBW (Red, Green, Blue, White) layout can be used which includes an additional fourth “white” sub-pixel (typically a sub-pixel without any color filter). If the pixel resolution and panel size remain the same, the sub-pixel apertures of an RGBW panel will be lower than for an RGB panel. However, as the white sub-pixel transmits all components of the backlight, its brightness can be approximated as the sum of the contributions by the red, green and blue filters thereby providing a potential doubling of the intensity of each color. This more than compensates for the reduced aperture and provides an effective aperture of each color which is typically around 50% higher than for the corresponding RGB panel, and thus can provide a total theoretical peak white brightness increase of 50%.
The use of multi-color backlights may thus not only provide an increased image quality but may also provide improved power efficiency. For example, RGBW panels can be particularly efficient if the single color backlight is replaced by colored backlight such as an RGB LED backlight. An example of such a display is shown in FIG. 1.
In particular, the use of a colored (e.g. LED) backlight, in addition to a better color reproduction, provides another important benefit in that it allows an independent control of R-, G-, and B-backlight channels. This may be used to substantially reduce the overall power consumption. For example, the LED channels that do not contribute a lot to image rendering can be dimmed thereby saving power.
This can be illustrated by FIG. 2 which illustrates two RGBW gamuts (with the 2D-projection on the R, G vector field being illustrated) for the same image content lacking saturated red colors. The color points of the image are represented by dots and FIG. 2 shows the color gamut for a white backlight compared to the reduced gamut that can be achieved by a multicolor backlight by reducing the backlight of the individual backlight channels to the lowest level that still allow all color points to be rendered. Thus, in the two examples the backlight is minimized at much as possible without incurring clipping. As can be seen the gamut induced by RGB backlight is more flexible and can be more accurately adapted to the specific image content thereby requiring less backlight resulting in reduced power consumption.
The determination of suitable colored backlight values for a given image is critical for both the image quality and the power consumption of the display. Unfortunately, this is a complex and resource demanding task as the impact of more than one sub-pixel must be considered for each backlight color. In particular, each primary color is dependent on at least two sub-pixels (typically the primary color sub-pixel and the white sub-pixel).
European Patent Application EP 06114488 and EP 07735967 proposes a technique for determining backlight intensities in such a scenario, and specifically for determining RGB backlight values for an RGBW display panel. In the publication, the backlight optimization problem is formalized as a search for the minimal backlight values that allow the picture content to be displayed without clipping artifacts. An efficient algorithm is provided for finding the backlight intensities. However, although a highly advantageous algorithm is proposed, it would be desirable for an even further improved approach. In particular, an approach having reduced computational demands, providing an improved performance, providing higher image quality, facilitating operation or implementation and/or providing improved performance would be advantageous.